Lead-free alloy

ABSTRACT

Lead-free Tin-Zinc alloy contains Manganese in the amount of about 0.001-0.9 wt. %. The alloy is suitable for use as a solder with commercially available fluxes. Various types of solders can be prepared from the alloy, like wire, cored wire, atomized powder, solder paste, thin sheet, ribbon foil, perform etc. The alloy has improved mechanical and electrical properties in comparison with conventional Tin-Lead alloys Sn9Zn and 63Sn37Pb.

FIELD OF THE INVENTION

[0001] The present invention relates to a Lead-free Tin-Zinc alloysuitable for use as a solder. By the term “solder” here is meant a metalalloy, which when melted and applied to the joint between metal objectsunites them without heating the objects to the melting point.

[0002] By virtue of the present invention the solder ensures strong andchemically stable joint between copper, brass, nickel, stainless steeland other frequently soldered metallic materials, which are used in avariety of applications including, but not limited to, electronics andgeneral purpose soldering applications.

[0003] The advantages of the solder of the invention comparing to theknown in the art Zinc-containing solders are associated with the factthat the present solder does not require active & special chemistryfluxes and therefore a wide range of commercially available fluxesincluding VOC free fluxes can be used with the solder of the invention.

[0004] Furthermore, the solder of the invention exhibits excellent flowand wetting behavior, and therefore is very efficient in the solderingapplications, where filling of tight capillary joints as well as looseones is required.

[0005] Moreover, the solder of the invention has improved corrosionresistance, since no corrosion of the substrate and oxidation of thepresent solder has been observed at the peripheral contact area betweenthe solder and the substrate.

[0006] The Lead-free alloy of the invention has improved electricalconductivity as compared with the conventional Tin-Lead alloy(Sn63Pb37), and it has an eutectic melting point of 199° C., which isrelatively similar to the melting characteristics, and is as close aspossible to the melting point of conventional Tin-Lead alloy (183° C.).

[0007] The alloy of the invention consists of, in weight %, about 3% toabout 13.4% Zinc, about 0.001% to about 0.9% Manganese and the balanceis Tin.

[0008] Presence of Manganese in the alloy improves its oxidationresistance and corrosion resistance and also decreases thesolidification range for a relatively wide range of compositions, thusrendering the manufacturing process of the alloy more flexible.

BACKGROUND OF THE INVENTION

[0009] Since the soldering industry is based mainly on the use ofTin-Lead eutectic, the obvious choice for its replacement would be aLead-free alloy with similar melting characteristics, i.e. with themelting point, which is as close as possible to 183° C. There areavailable some Lead-free alloys, which have melting point in thetemperature range 180-200° C. and which in general are based on amixture of Tin with other elements such as Zinc, Bismuth and Indium. TheTin-9% Zinc alloy is the only one, which has eutectic in thistemperature range, melting at 198.5° C. Examples of some other availableternary alloy are shown in the following table.

Example of Lead-free solder alloys melting at 180-200° C. (Report ofBritish DTI, 1999 “An analysis of the current status of Lead-freesoldering”)

[0010] Composition Melting Range Alloy System (wt. %) (° C.) Sn—ZnSn—9Zn 198.5 Sn—Bi—Zn Sn—87Zn—3Bi 199-189 Sn—Bi—In Sn—20Bi—10In 143-193

[0011] Unfortunately all these alloys possess certain notabledisadvantages. In particular alloys with significant levels of Indiumare very expensive, and are prone to the formation of a low temperaturephase in the alloy. This phase melts in the region of Tin-Indiumeutectic at 117° C. and causes cracking in the soldered joints. Tin-Zincbased alloys can be made with melting temperature very close to that ofthe SnPb eutectic, but the presence of Zinc causes many problemsassociated with its reactivity, i.e. soldering with these alloysrequires using of special and relative active fluxes, the alloys exhibitexcessive drossing and oxidation.

[0012] It should be mentioned that despite the fact that numerousLead-free alloys for use in soldering have been devised there is stillfelt a need for a new and improved alloy, which is inexpensive,convenient in manufacturing and ensures efficient and reliablesoldering.

THE OBJECTS OF THE INVENTION

[0013] The object of the present invention is to provide for a new andimproved Lead-free alloy enabling sufficiently reduce or overcome theabove-mentioned drawbacks of the known in the art solutions.

[0014] The main object of this invention is to provide for a Lead-freealloy that can be used as a solder.

[0015] The further object of the invention is to provide for a solder,which exhibits high strength and ensures chemically stable joint betweensoldered metallic materials, including those frequently used in theelectronic industries.

[0016] Still further object of the invention is to provide for a solder,which has a eutectic melting point below 200° C. and has enhancedmechanical-physical properties as compared to the conventional Tin-Leadalloy (63Sn37Pb).

[0017] Another object of the invention is to provide for a solder, whichdoes not require special and relative active fluxes and which is notprone to excessive oxidation.

[0018] Still further object of the invention is to provide for aLead-free alloy, which is suitable for use as a solder in variousphysical forms, comprising wire, cored wire, atomized powder, paste,sheet, ribbon, foil, preform etc.

[0019] In accordance with the invention the above and other objects andadvantages can be achieved with the composition of the Lead-free alloyconsisting (in weight %) essentially of Tin, Zinc and up to 0.9% ofManganese.

[0020] In one of the preferred embodiments the composition of theLead-free alloy consists (in weight %) essentially of about 3.5-13.4% ofZinc, about 0.001-0.9% of Manganese and the balance is Tin.

[0021] In still further preferred embodiment the composition of theLead-free alloy consists (in weight %) essentially of about 7.5-8.5% ofZinc, about 0.4-0.5% of Manganese and the balance is Tin.

[0022] In yet another preferred embodiment the composition of theLead-free alloy consists (in weight %) essentially of about 7.5-9.0% ofZinc, about 0.001-0.15% of Manganese and the balance is Tin.

[0023] According to the additional preferred embodiment the compositionof the Lead-free alloy consists (in weight %) essentially of about8.3-8.5% of Zinc, about 0.005-0.02% of Manganese and the balance is Tin.

[0024] In still further preferred embodiment the Lead-free alloycomprises a solder, which is selected from the group consisting of wire,cored wire, atomized powder, paste, sheet, foil and preform.

[0025] For better understanding of the present invention as well of itsadvantages, reference will now be made to the following description ofits embodiments with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIGS. 1,2 show compositions of the present alloy andliquidus-solidus temperatures.

[0027]FIG. 3 is binary phase diagram of Tin-Zinc.

[0028]FIG. 4 is binary phase diagram of Manganese-Zinc.

[0029]FIG. 5 is dependence of Liquidus-Solidus temperature of thepresent alloy on the amount of Zn and Mn.

[0030]FIGS. 6-9 show microstructures of various compositions of thepresent alloy and of the conventional Tin-Zinc based alloy.

[0031]FIG. 10 shows wetting angles of various compositions of thepresent alloy and of the conventional Tin-Zinc based alloy.

[0032]FIG. 11 illustrates behavior of various compositions of thepresent alloy with different fluxes on copper substrate.

[0033]FIG. 12 shows shear strength of copper joints measured byT-peel-test.

[0034]FIG. 13 depicts physical & mechanical properties of thecomposition No.8.

[0035]FIG. 14 shows surface morphology of the present alloy manufacturedin the form of atomized powder.

[0036]FIGS. 15-16 show results of the Auger spectrographic measurementstaken on the surface of the atomized powder.

[0037]FIG. 17 shows results of the EDS analysis made on the surface ofthe atomized powder.

[0038]FIG. 18 shows wetting angles of various compositions referring toatomized powder.

[0039]FIG. 19 illustrates soldering behavior of the compositionsreferring to atomized powder

DETAILED DESCRIPTION OF THE INVENTION

[0040] It has been unexpectedly experimentally revealed, that variouscompositions based on Tin-Zinc alloy, which contain small alloyingaddition, mainly Manganese (Mn), have improved properties that renderthe alloy very suitable and efficient for use in soldering inelectronics, automotive industries and in general purpose solderingapplications.

[0041] In general the present invention concerns an alloy consistingessentially of, in weight %:

[0042] Zinc (Zn)—about 3.4% to about 13.4%.

[0043] Manganese (Mn)—about 0.001% to about 0.9%.

[0044] Tin (Sn)—balance.

[0045] Lead-free alloy of the invention can be prepared by anytechnique, which effectively mixes the alloy constituents taken inamounts referring to compositions, which will be shown further. Suchtechniques include, but are not limited to, melting a mixture ofelements or partially alloyed elements, preferably in an inertatmosphere.

[0046] The different compositions of the present alloy are summarized inFIGS. 1 & 2, which respectively refer to two different groups ofembodiments.

[0047] The first group of embodiments, i.e. the compositions No.No. 1-13refer to those alloy compositions, which are suitable for manufacturingof solders at a low cooling rates, namely in the order of magnitude ofnot more than 1 degree C. per second. The solders manufactured fromthese compositions at low cooling rates are in a stable condition. Amongsuch solders are wires, ingots, sheets, foils, ribbons, preforms, etc.

[0048] The second group of embodiments, i.e. the compositionsNo.No.15-24 refer to those alloy compositions, which are suitable formanufacturing of solder at a high cooling rate, namely in the order ofmagnitude of at least 10 degrees C. per second. The solders prepared athigh cooling rate from the compositions of the second group are in ametastable condition. In particular these compositions is suitable formanufacturing the solder in the form of atomized powder. The atomizedpowder can be also used for preparation of soldering paste.

[0049] Now with reference to FIGS. 5-9 the microstructurecharacteristics and the liquidus-solidus temperature of the compositionsreferring to the first group of embodiments will be explained.

[0050] As seen in FIG. 3 the binary Tin-Zinc alloy has a eutecticmelting temperature of 198.5° C. Small amounts of Manganese and Zinctaken at a constant ratio of 1 atom of Manganese to 13 atoms of Zincwere added to Tin. The selected ratio corresponds to the stoichiometricratio between Manganese and Zinc in the ξ phase presenting in the Mn—Znphase diagram as shown in FIG. 4.

[0051] With reference to FIG. 5 the presence of Manganese decreases theliquidus-solidus temperature and it can be seen that the temperaturerange corresponding to the alloy compositions No.No.1-13 (3.5-13.5% wtof Zinc) is about 198° C. The data, presented in FIG. 5 lies within theaccuracy of the DTA measurements, which is ±0.75° C. According to theDTA measurements, the alloy compositions No.No.7-13 can be defined aspseudo eutectic compositions.

[0052] For the purpose of accurate determination of the eutectic meltingpoint, a metallographic examination of the compositions No.No.1-13 hasbeen carried out. Referring now to FIG. 6 it is seen that thecomposition No.8 has the most homogenous eutectic microstructure. Forthe purpose of comparison, metallographic specimens of the conventionalbinary Sn-9Zinc alloy (eutectic composition) and Tin-Zinc-Manganesealloy of the invention with the same Zinc content were prepared.

[0053] As seen in FIGS. 7-9 the microstructure of the binary eutecticalloy is not homogenous and the microstructure of the ternary alloyconsists of a eutectic phase and acicular phases of Zinc.

[0054] By adding of about 0.5% wt of Manganese, the eutectic compositionof binary Tin-Zinc alloy has been shifted to the content of 7.7-8.3% wtof Zn.

[0055] By virtue of the addition of Manganese in the amount of about0.5% wt it was possible to reduce the liquidus temperature withoutchanging the eutectic temperature (see FIG. 5) and thus to create arange of Tin-Zinc compositions (compositions No.No.8-13).

[0056] The wetting capability of the new alloy was examined withreference to the conventional eutectic Tin-Zinc alloy (Sn9Zn).

[0057] The wetting examination was carried out on a copper substrateetched by nitric acid (50%) and sulfuric acid (5%). The examination hasbeen carried out at 70° C. by dipping in the VOC Free WS770 flux.

[0058] The substrate was heated by an electrical plate to a temperatureof 230° C., after which a few alloy particles were put on the substrateuntil creating a melting pool.

[0059] After cooling down, metallographic specimens were prepared andwetting angle between the drop and the substrate was measured.

[0060] In FIG. 10 the wetting angles of different compositions of thenew alloy as well of the eutectic Tin-Zinc alloy (Sn9Zn), are shown.

[0061] The wetting angles of the most alloy compositions are good (<45°)and better than the wetting angle of the eutectic alloy (Sn9Zn).

[0062] The solderability evaluation of the compositions referring to thepresent alloy has been carried out on various substrates and withdifferent fluxes. Results of the evaluation are summarized in FIG. 11.From the results follows, that:

[0063] all compositions exhibit good solderability on a copper substratewith the Rosin RMA202 flux

[0064] all composition except the composition No. 14 and the binaryalloy Sn9Zn exhibit good solderability on a copper substrate with theWater Washable X20 flux and R103-OLI flux. The composition No. 14 andthe binary alloy Sn9Zn exhibit fair solderability on a coper substratewith the Water Washable ARAX Inorganic flux

[0065] the compositions No.No.6-13 exhibit either good or fairsolderablity on a copper substrate with the No Clean fluxes

[0066] all composition except the composition No.14 and the binary alloySn9Zn exhibit good solderability on a copper substrate with the VOC freeflux.

[0067] It has been also found, that in contrast to the binary Sn9ZNalloy the compositions of the invention could be soldered on a nickelsubstrate with the fluxes, from the group of Rosin, VOC and WaterWashable fluxes and on a stainless steel substrate using the WaterWashable ARAX flux.

[0068] According to the above results, it can be clearly seen that thepresent alloy has the required properties enabling its use as a solder.

[0069] In order to evaluate the advantages of the new alloy, as comparedto the other conventional Tin-Lead alloy some mechanical properties andelectrical conductivity of the composition No.8 were examined. In FIG.13 the measured properties of the present alloy are compared with thedata published in the Metals Handbook (pub. by ASM 10^(th) Ed., V6 p.966-967) for the conventional alloy 63Sn37Pb alloy. It can be readilyappreciated, that the mechanical and electrical conductivity propertiesof the new alloy are better than those of the conventional Tin-Leadalloy.

[0070] Specimens for shear strength testing of the soldered joint wereprepared from the compositions of the invention and from theconventional 63Sn37Pb alloy.

[0071] The specimens were prepared and tested according to the ASTMD1876 standard (Standard Test Method for Peel Resistance of Adhesive,T-peel test).

[0072] The obtained results are shown in FIG. 12 and it can be seen,that shear strength of joints referring to the compositions No.No.7, 8,11 and 12 is higher than of the joint obtained with conventionalTin-Lead alloy.

[0073] Now the compositions of the second group of embodiments will bedisclosed.

[0074] By using an atomizing process with a high cooling rate it ispossible to prepare the alloy in a metastable condition and in the formof a powder. An example of such a powder is shown in FIG. 14, depictingthe morphology of the atomized powder prepared from the compositionNo.22.

[0075] It has been found, however, that the wetting properties andsolderability properties of the atomized powder are deteriorated due tothe precipitation of Manganese and therefore its content should bereduced, comparing to the compositions of the first group, which wereprepared at a low cooling rate and are in a stable condition.

[0076] To prove this assumption the atomized powder was examined by theAuger spectroscopy and by the EDS analysis.

[0077] As best seen in FIG. 14 the surface morphology of the powderparticles is defined by a “grained” texture. In FIG. 15 are seen twolocations on the powder particle, designated as P1 and P2 and referringrespectively to the “grain” itself and to the “grain boundary”. Thecontent of Manganese in both locations was measured by the Augerspectroscopy. The results are seen in FIGS. 16a, 16 b. From the resultsfollows, that the content of Manganese in the above locations (in atomic%) is 0.8 and 4.4 respectively. It should be noticed that the atomicabsorption analysis had shown that the composition No.22 contains only23 ppm of Manganese.

[0078] Results of the EDS analysis of the powder surface are shown inFIG. 17. The analysis has been carried out with the excitation volumedepth 5 micron. The analysis hadn't revealed any sign of Manganese atall.

[0079] From the obtained results follows that in the alloy, which isproduced at a high cooling rate and is in a metastable condition theprecipitation of Manganese occurs mostly on the particles surface andnot in the bulk as in the case of an alloy produced at a slow coolingrate. Therefore it would be sufficient if the Manganese content in thesolders produced at a high cooling rate is less comparing with thesolders produced at a low cooling rate. We found that the Manganesecontent of about 0.001-0.15% would be a sufficient range.

[0080] In FIG. 18 are shown wetting angles of different compositions ofthe present alloy prepared in the form of atomized powder and of theconventional eutectic Sn9Zn alloy. It can be readily appreciated thatwetting characteristics of the present alloy are better than of theconventional alloy.

[0081] Results of the solderability evaluation of the atomized alloys oncopper substrate, with different fluxes are summarized in FIG. 19. Itcan be seen that all compositions exhibit good solderability with theRosin, Water Washable and VOC fluxes. The compositions NoNo.19-24exhibit either good or fair solderability with the No Clean fluxes.

[0082] Two solder pastes were prepared from the atomized powder of thecomposition No.22 mixed with a flux paste medium. The flux paste mediumin the first paste was RMA 291 flux and in the second paste NC 291 AX.The pastes were printed on a PCB that was soldered through a reflowoven. The reflow profile was a conventional one with the peaktemperature of 235° C. The solder pastes exhibited good wetting andsolderability characteristics.

SUMMARY

[0083] It is evident that the present alloy has improved properties incomparison with the conventional eutectic Tin-Zinc alloy (Sn9Zn) and canbe used as a solder.

[0084] By virtue of the improved properties it is possible to use thepresent alloy with different types of commercial fluxes and no sign ofcorrosion and oxidation is observed in the soldered assembly parts.

[0085] The new alloy also has improved mechanical and electricalproperties as compared to the conventional Tin-Lead alloy (63Sn37Pb).

[0086] The solder of the invention is amenable to manufacture as a solidround wire and is also suitable for solder wire cored with the Rosin orother commercially available organic or inorganic fluxes.

[0087] The solder can also be manufactured in other physical forms, likethin sheet, foil, ribbon, preforms or atomized powder, suitable formixing with a flux to provide a solder paste.

[0088] It should be appreciated that the present invention is notlimited by the above described embodiments and that one ordinarilyskilled in the art can make changes and modifications without deviationfrom the scope of the invention as will be defined below in the appendedclaims.

[0089] For example, despite the alloy has been disclosed in theforegoing description in connection with the soldering application,nevertheless the other applications of the alloy should be consideredalso within the scope of the invention. Examples of such alternativeapplications are manufacturing of coatings on cans made of stainlesssteel, copper based alloys etc., casting or powder metallurgymanufacturing of small details for electronic industry etc.

[0090] It should also be appreciated that features disclosed in theforegoing description, and/or in the foregoing drawings, and/orexamples, and/or tables, and/or following claims both separately and inany combination thereof, be material for realizing the present inventionin diverse forms thereof.

[0091] When used in the following claims the terms “comprise”,“contain”, “have” and their conjugates mean “including but not limitedto”.

1. A Lead-free alloy comprising (in weight %) essentially Tin, Zinc andup to 0.9% of Manganese.
 2. The Lead-free alloy as defined in claim 1,comprising (in weight %) essentially about 3.5-13.4% of Zinc, about0.001-0.9% of Manganese and the balance is Tin.
 3. The Lead-free alloyas defined in claim 2, comprising (in weight %) essentially about7.5-8.5% of Zinc, about 0.4-0.5% of Manganese and the balance is Tin. 4.The Lead-free alloy as defined in claim 2, comprising (in weight %)essentially about 7.5-9.0% of Zinc, about 0.001-0.15% of Manganese andthe balance is Tin.
 5. The Lead-free alloy as defined in claim 4,comprising (in weight %) essentially about 8.3-8.5% of Zinc, about0.005-0.02% of Manganese and the balance is Tin.
 6. The Lead-free alloyas defined in claim 1, said alloy comprising a solder.
 7. The Lead-freealloy as defined in claim 6, in which said solder is selected from thegroup consisting of wire, cored wire, atomized powder, paste, sheet,foil and preform.